When a capacitor is charged, an inrush current is generated at the time of applying a voltage. As a method for suppressing the inrush current, a method that reduces the peak value of the inrush current by a resistor which is inserted into a power supply line is disclosed. However, in the case of reducing the inrush current by the resistor, taking the allowable power dissipation of the resistor into consideration, it is necessary to use a so-called high-power resistor, and this leads to an increase in cost and size. Therefore, in order to suppress such an increase in cost and size, a constant current circuit that is configured with a small bipolar transistor has been used (for example, JP 2000-289452A (Reference 1) and JP 2012-60697A (Reference 2)).
In the Reference 1, an energization device that charges the capacitor of an electric car air conditioning apparatus is disclosed. In the energization device, the emitter terminal of an output transistor is connected to the base terminal of a control transistor, and the base terminal of the output transistor is connected to the collector terminal of the control transistor. A resistor and a zener diode are provided between the base terminal and the emitter terminal of the control transistor. The constant current value is set by the sum of the base-emitter voltage and the zener voltage and the resistor.
In the Reference 2, a constant current circuit for charging is disclosed. In the constant current circuit for charging, the collector terminal of a first transistor is connected to the base terminal of a second transistor, and the base terminal of the first transistor is connected to the emitter terminal of the second transistor. A resistor and a diode are provided between the base terminal and the emitter terminal of the first transistor, and the constant current value is set by the difference between the base-emitter voltage and the forward voltage of the diode and the resistor.
According to the circuits described in the References 1 and 2, the charging current of the capacitor can be restricted to a predetermined current value. Therefore, the inrush current can be suppressed. However, when the voltage of the positive terminal of the capacitor becomes high according to the charging and the charging current becomes excessively smaller than the predetermined current value, the output transistor in the Reference 1 is turned-off, and the second transistor in the Reference 2 is turned-off. In this case, the subsequent charging current becomes small to the extent of the leakage current of each transistor (for example, μA order), and this leads to significantly increase the time until the inter-terminal voltage of the capacitor reaches to the power supply voltage.